1. Field of the Invention
The present invention relates to a reflection type compound prism which reduces a height of an optical system, and a thin optical pickup apparatus employing the reflection type compound prism.
2. Description of the Related Art
Referring to FIG. 1, a conventional optical pickup apparatus has an optical system in which a light beam emitted from a light source 2 proceeds horizontally, is reflected by a reflection mirror 5 having a reflection surface 5a inclined by 45°, proceeds in a direction of a height h, and is condensed by an objective lens 3 to be focused on an optical disc 1.
A recording density of the optical disc 1 is determined by a size of a light spot formed by the objective lens 3 on a recording surface of the optical disc 1. The size of the light spot decreases as the wavelength of light used becomes shorter and an NA (numerical aperture) number of the objective lens 3 becomes greater. Assuming that an effective diameter and an effective focal length of a light beam input to the objective lens 3 are EPD and EFL, respectively, and an incident angle of a light beam focused on the recording surface of the optical disc 1 is θ, NA=sin θ and θ=arctan{EPD/(2EFL)}. Thus, in order to maximize an effective NA number to minimize the size of a light spot under the above given circumstances, the light beam incident on the objective lens 3 should have a diameter corresponding to the effective diameter of the objective lens 3.
The thickness, in particular, the thickness in the direction of the height h, of the conventional optical pickup apparatus having the optical structure as shown in FIG. 1 is determined by the diameter of the light beam input to the objective lens 3, a thickness of the objective lens 3, the thickness in the direction of a height of an actuator (not shown) for driving the objective lens 3, a size of the reflection mirror 5, and an angle between the reflection mirror 5 and a horizontal plane which is perpendicular to the direction of height.
However, in the conventional optical pickup apparatus having the above structure, to increase the size of a light beam input to the objective lens 3 so as to obtain a desired NA number, the reflection mirror 5 must be made larger. Thus, it is difficult to manufacture a compact/thin optical pickup apparatus.